1. Field of the Invention
This invention relates to a plant and a method for preparing a refuse derived solid fuel (RDSF).
2. Description of the Related Art
EP-A-0 930 353 discloses a solid fuel composition having a bulk density of less than 0.6 g/cm3, consisting of a mixture comprising a first fraction consisting of the dry fraction deriving from municipal solid waste (MSW), a second fraction consisting of elastomeric material and a third fraction consisting of non-elastomeric polymer material. The ratios between the fractions vary as the desired calorific value varies.
EP-A-0 953 628 discloses a combustible composition which burns instantaneously comprising from 40 to 95% by weight of a fossil fuel and from 60 to 5% by weight of a non-fossil solid fuel selected from the group comprising municipal solid waste (MSW), elastomeric polymer material and non-elastomeric polymer material and mixtures thereof. In the combustion plants disclosed in EP-A-0 953 628 the non-fossil solid fuel is stored in a silo from which it is then delivered to the burner.
The Applicant has first noted that direct feed of the RDSF from the production plant to the combustion plant without storage stages is not normally practicable, since the flow rates required by the latter are generally different from those provided by a plant for the production of RDSF. It should be noted that typically a combustion plant works continuously for 24 hours a day, while the output of RDSF is subject to its production methods, in particular the material collection, which is inherently of a discontinuous nature, and, for example, may be carried out in two shifts a day, for about 16 hours per day.
The Applicant has also noted that the storage of a RDSF consisting of the dry fraction deriving from municipal solid waste (MSW) when mixed with elastomeric polymer material and/or thermoplastic (non-elastomeric) polymer material gives rise to a number of disadvantages.
For the purposes of the present description, by “dry fraction deriving from municipal solid waste (MSW)” is meant a fraction obtained by treating MSW as such by: removal of the wet fraction consisting of putrescible organic material, separation of metal materials, shredding and possibly, if necessary, drying.
A first disadvantage of the storage of RDSF derives from the fact that during storage the aforesaid materials, in particular the dry fraction deriving from the MSW, tend to aggregate, forming compact agglomerations. This could be avoided by using large storage containers provided with complex mixing equipments, for example paddle machines, capable of keeping the stored materials under constant stirring.
However, in addition to resulting in high initial plant costs and then in operating costs for the same, and considerable problems associated with the use of space on account of the large storage volumes required, such storage containers could have the further disadvantage of not hindering but instead favouring separation of the heavier and/or more voluminous particles from the lighter and/or less voluminous particles as a consequence of the action of the mixing equipment itself.
This would therefore result in a non-uniform mixture in terms of composition and density which, if fed to a burner, would cause considerable instability in the combustion process and in particular would not be capable of ensuring a calorific value which was substantially constant over time.
A further problem derives from the fact that it is frequently necessary to vary the composition of the RDSF produced in relation to use requirements in a combustion process, in particular the desired calorific value. It would therefore be necessary to provide a plurality of containers for the storage of different types of RDSF required by the users.